1. Field of the Invention
The present invention relates generally to a light control apparatus for an image sensing device and, more particularly, to an apparatus for controlling an amount of light incident on an image sensing device by adjusting size of a diaphragm or light transmittance of the diaphragm.
2. Description of the Related Art
There have been recent demands for miniaturization and high performance of image sensing devices such as digital video cameras, digital still cameras, and smart-phones. To meet such demands, in other words, to make the image sensing device smaller, a miniaturized and high-density integrated CCD (Charge Coupled Device) or a high-density CMOS (Complementary Metal-Oxide Semiconductor) chip is used as the image sensing tool.
Also, to miniaturize the size of the image sensing device, it is necessary to reduce the size of both lens and diaphragm. The usual optic diaphragm applied to a general image sensing device is an IRIS system. The IRIS system in which triangular diaphragm blades partially overlap each other and are arranged to make a round shape in the lens aperture, mechanically or electrically controls the diaphragm blades to adjust the size of the round hole, whereby it controls the quantity of light incident on the image sensing tool.
To miniaturize the size of the IRIS system while retaining the performance of the system presents technical difficulties. These technical difficulties will be specifically described herein.
To reduce the size of the IRIS system, it is necessary to reduce both the number of the diaphragm blades and the size of each individual diaphragm blade. Reducing both the number and size of the diaphragm blades leads to a reduction of the aperture diameter of the diaphragm of the IRIS system and to a limit of the number of levels upon the quantity of incident light. Therefore, it is difficult to get clear images in both dark and bright scenes by using miniaturized IRIS system. Also, as the light diffraction is intensified in the peripheral regions between the miniaturized diaphragm blades, the image recognized by the image sensor may be transformed. Consequently, it is not easy to precisely produce the miniaturized diaphragm blades and but also to minimize the IRIS system using the diaphragm blades.
Recently, using LCD (Liquid Crystal Display) technology, a method of implementing an electro-optic shutter or a method of controlling light transmittance is being proposed to solve the above problem [Patent Documents 1 to 11].
Patent Documents 1, 2, 3, 5, and 11 describe the method for applying an LCD shutter as an optical shutter to the image sensing device. However, those methods present problems that, due to the polarizers attached on the LCD panel, the light transmittance of the LCD shutter at clear state is too low to get clear images in dark circumstances, and if the polarized light is incident on the image sensing device, the image quality may be compromised.
In another related art, the suggested method involves electrically controlling the size or shape of the diaphragm by applying an LCD to the optical system of the image sensing device [Patent Documents 7, 11]. However, the method has problems including the loss in light transmittance due to a polarizing plate applied to the LCD, the transformation of an image attributable to the strong interaction between incident light and the polarizing plate, and difficulties in maintaining uniform light transmittance of the diaphragm due to the optical non-uniformity of electrode patterns that are formed to drive a plurality of pixels of the LCD. Therefore, the method has not been put to practical use.
These days, for improving image quality of digital cameras and smartphone cameras, a multi-frame image sensing method is applied, which gets multiple images for one object by changing the setting value of the camera over a short period of time and selects the best one among the multiple images [Reference to Korean Patent Publication No. 2012-0016089].